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School of Materials Science and Engineering

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College of Engineering

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Nonlinear block copolymer nanoreactor-enabled precision synthesis of metal oxide nanoparticles with controlled dimensions and compositions and investigation into their electrocatalytic and magnetic properties

(Georgia Institute of Technology, 2020-07-27) Harn, Yeu Wei

Colloidal nanocrystals carry a wide array of intriguing features, thereby rendering a rich diversity of applications in optics, electronics, optoelectronics, catalysis, sensors, energy conversion and storage, nanotechnology, biotechnology, among other areas. Despite copious past works on development of synthetic approaches, the ability to craft functional nanocrystals with precisely controlled compositions and dimensions under mild reaction condition and facile purification process is comparatively few and limited in scope. In this thesis, we developed a general and robust star-like block copolymer nanoreactor strategy for precision synthesis of an assortment of metal oxide nanoparticles controlled dimensions, compositions and surface chemistry, followed by investigation into their size- and composition-dependent electrocatalytic and magnetic properties. We first successfully synthesize both amphiphilic and hydrophilic star-like diblock and triblock copolymers, which can subsequently function as nanoreactors for templating the precision synthesis of inorganic nanoparticles. In particular, the molecular weights and molecular weight distribution of star-like block copolymers can be readily controlled with the addition of linear initiator as living radical polymerization is achieved. Furthermore, the dual pH-responsive behavior of double hydrophilic star-like diblock copolymer is found, suggesting the potential application as polymer nanocarriers for control-release of drugs. By capitalizing on amphiphilic star-like diblock copolymer as nanoreactors, perovskite oxide (i.e., BaTiO3, PbTiO3 and doped BaTiO3), La-based perovskite and layered perovskite (i.e., LaFeO3, LaMnO3 and La2CoO4) and magnetic spinel ferrite (i.e., CoFe2O4, MnFe2O4 and NiFe2O4) nanoparticles with controllable size can be obtained. Due to the unique compositional versatility and structural stability of perovskites, investigation into electrocatalytic performance of all the as-synthesized nanoparticles with perovskite and layered perovskite structure is conducted. First, we report the scrutiny of size- and dopant-dependent oxygen reduction reaction (ORR) activities of an array of pristine BaTiO3 and La- or Co-doped BaTiO3 nanoparticles. The ORR activities are found to progressively decrease with the increasing size of BaTiO3 NPs because of increased active sites and electroconductivity. On the other hand, La- and Co-doped BaTiO3 NPs display markedly improved ORR performance over the pristine counterpart, which can be attributed to the reduced limiting barrier and the possibly-increased conductivity that are verified by density functional theory-based first principle calculations. Second, the bifunctional electrocatalytic activity of LaFeO3, LaMnO3 and La2CoO4 nanoparticles for both ORR and oxygen evolution reaction (OER) have been revealed. Among all, layered La2CoO4 perovskite nanoparticles exhibit remarkable activity and excellent stability (i.e., with the oxygen activity (ΔE) of 0.88 V when D = 8.5 nm), which is greater than that of the previously reported perovskite and perovskite derivative electrocatalysts. The superior catalytic performances of layered La2CoO4 perovskite NPs may be resulted from highly active lattice oxygen and an increased concentration of hydroxyl groups, corroborated by the pH-dependent OER behavior and XPS of O 1s results, respectively. Finally, three representative spinel ferrites (i.e., MnFe2O4, NiFe2O4, CoFe2O4) nanoparticles with tunable size are prepared. The size- and composition-dependent magnetic properties are explored and discussed. Moreover, by changing the outer block of the star-like diblock copolymer, hydrophilic poly(ethylene glycol)-ligated spinel ferrite nanoparticles can be acquired, leading to possible biomedical applications. By extension, amphiphilic star-like block copolymer nanoreactor strategy is anticipated to enable the crafting of an exciting variety of other functional nanomaterials with tailored sizes, compositions and surface chemistry. As such, the fundamental correlations between morphologies, compositions and properties of judiciously designed nanomaterials can be elucidated, rendering the optimized materials performance.
Triboelectric nanogenerators for energy harvesting, self-powered sensing and high voltage applications

(Georgia Institute of Technology, 2020-07-06) Zhang, Steven L.

As we are currently entering an age of Internet of Things (IOTs), where electronic devices are able to perceive, interpret, and judge for themselves, intensive research efforts are needed on fabricating energy sources to power these electronic devices and on developing different sensor networks. Currently, different methods, such as using solar, thermal, nuclear, and mechanical motions, have been used to harvest the energy required to power these sensors. For the latter, triboelectric nanogenerators (TENGs), which are developed in Professor Zhong Lin Wang’s group in 2012, is one of the best choices to harvest mechanical vibrations, due to triboelectrification is an universal and ubiquitous effect with an abundant choice of materials. Also, not only the TENG could be used as an energy harvester, it could also be utilized as a self-powered active sensor, which is able to sense characteristics of different mechanical motions, expanding its ability to operate as a sensing network. Furthermore, TENGs due to their high voltage characteristics, have been utilized in various high voltage applications recently. In this thesis, three main research area relating TENGs are focused. The first is to develop a more selective and sensitive self-powered active triboelectric sensor through the use of novel materials. Also, by utilizing, signal processing and machine learning algorithms, different applications of self-powered sensors are investigated. The second is to expand the use of TENGs to harvest energy more effectively in harsh environments. This is done by investigating new theory of dielectric loss effects on TENG in different harsh environments. The third is to use TENG as a high voltage source for novel applications, such as a self-powered sensing system and a self-powered personal security device.
Applications of piezotronics and piezo-phototronics

(Georgia Institute of Technology, 2019-05-31) Zou, Haiyang

Third-generation semiconductor materials have superior performance with high voltage resistance, high frequency, high efficiency, high-temperature resistance and high radiation resistance, that they look to be the “core” of a new generation of information technology, energy saving, and smart manufacturing. They can have broad application prospects in many fields, and have attracted the attention of governments, industries and research communities all over the world and achieved rapid development. The typical materials such as GaN and ZnO simultaneously exhibit piezoelectric, semiconducting and photoexcitation properties. The piezoelectric polarization charges can be utilized to control/tune the charge carrier transport characteristics in these materials (piezotronic effect), and also used to tune the generation, transport, separation and/or recombination of charge carriers (piezo-phototronic effect). The coupling of these properties in these materials has resulted in both novel fundamental phenomenon and unprecedented device application, leading to the increasing research interests in the emerging field of piezotronics and piezo-phototronics. Functional electronic and optoelectronic devices are presented to illustrate the practical applications of the piezotronic and piezo-phototronic effects. Fundamental physics about the piezotronics and piezo-phototronics are further studied in this work. This will help to develop a full understanding of piezotronics and piezo-phototronics, and it also enables the development of the better performance of optoelectronics. By applying the two effects in a wide range of electronics/optoelectronics, we have shown they are effective approaches to modify the physical properties in piezoelectric semiconductors and a useful tool to study the physical model in a complex system.
High performance triboelectric nanogenerator and its applications

(Georgia Institute of Technology, 2019-05-24) Wu, Changsheng

Fundamental performance-limiting factors of triboelectric nanogenerator were studied and various strategies, such as the optimization of materials, structures and operation environment, were investigated to enhance its electrical outputs. Furthermore, the application of triboelectric nanogenerator in three major fields, including micro/nano power sources, self-powered sensors, and direct high voltage power sources, was explored. Self-powered electrically-assisted transdermal drug delivery driven by biomechanical motions was demonstrated for non-invasive, on-demand drug administration with feedback control. Self-powered wireless optical transmission of mechanical agitation signals was proposed to solve the issue of power supply for optical wireless communications. Smart keyboard with active pressure sensing was developed for keystroke dynamics-based cyber security. The high voltage of triboelectric nanogenerator was successfully applied to drive field emission of electrons and electrohydrodynamic jet printing, with unique merits of low cost, enhanced safety and portability. With solid understanding of both the fundamentals and applications, a roadmap is proposed for the research and commercialization of triboelectric nanogenerator in the next 10 years. This work not only provides insights and solutions for developing high performance triboelectric nanogenerator, but also broadens its application in a variety of multidisciplinary fields that have a huge impact on people’s daily life in the era of Internet of Things.
Triboelectric nanogenerators

(Georgia Institute of Technology, 2016-02-23) Chen, Jun

With the threatening of global warming and energy crises, searching for renewable and green energy resources with reduced carbon emissions is one of the most urgent challenges to the sustainable development of human civilization. In the past decades, increasing research efforts have been committed to seek for clean and renewable energy sources as well as to develop renewable energy technologies. Mechanical motion ubiquitously exists in ambient environment and people’s daily life. In recent years, it becomes an attractive target for energy harvesting as a promising supplement to traditional fuel sources and a potentially alternative power source to battery-operated electronics. Until recently, the mechanisms of mechanical energy harvesting are limited to transductions based on piezoelectric effect, electromagnetic effect, electrostatic effect and magnetostrictive effect. Widespread usage of these techniques is likely to be shadowed by possible limitations, such as structure complexity, low power output, fabrication of high-quality materials, reliance on external power sources and little adaptability on structural design for different applications. In 2012, triboelectric nanogenerator (TENG), a creative invention for harvesting ambient mechanical energy based on the coupling between triboelectric effect and electrostatic effect has been launched as a new and renewable energy technology. The concept and design presented in this thesis research can greatly promote the development of TENG as both sustainable power sources and self-powered active sensors. And it will greatly help to define the TENG as a fundamentally new green energy technology, featured as being simple, reliable, cost-effective as well as high efficiency.
Piezotronics/piezo-phototronics: Devices and applications

(Georgia Institute of Technology, 2016-02-22) Yu, Ruomeng

Piezoelectric effect has been widely used in electromechanical sensing, actuation and energy harvesting, which produces polarization charges under mechanical deformation in materials lacking inversion symmetry or with polarization domains. Conventional piezoelectric materials such as PZT and PVDF are electrically insulating and hence not feasible for constructing functional electronics or optoelectronics. The effect of mechanically-induced polarization on electronic and optoelectronic processes of charge carriers in piezoelectric materials has therefore been long overlooked. Semiconductor materials such as ZnO, GaN and CdS with wurtzite or zinc blende structures also possess piezoelectric properties but are not as extensively utilized in piezoelectric sensors and actuators as PZT due to their relatively small piezoelectric coefficients. The coupling of piezoelectric polarization with semiconductor properties in these materials has resulted in both novel fundamental phenomenon and unprecedented device applications, leading to the increasing research interests in the emerging field of piezotronics and piezo-phototronics. The basic of piezotronics and piezo-phototronics lies in the fact that strain-induced polarization charges at interface can effectively modulate the local band structure and hence the charge carrier transport across local junctions/contacts by exerting substantial influence on the concentration/distribution of free carriers and interfacial electronic charged states in the device. Fundamental physics about the piezotronics and piezo-phototronics are systematically illustrated at first in this dissertation. Functional electronic/optoelectronic devices based on piezoelectric semiconductor materials are presented to demonstrate the practical applications of the piezotronic and piezo-phototronic effects, including nanowire/microwire transistors, nanowire logic circuits, bio/chemical sensors and photo detectors. By successfully applying the piezotronic and piezo-phototronic effects in a wide range of electronics/optoelectronics, we have shown the universality of these two effects to be utilized in various practical applications as effective approaches to modify the physical properties of charge carriers in piezoelectric semiconductors.
Theory of triboelectric nanogenerators for self-powered systems

(Georgia Institute of Technology, 2016-02-22) Niu, Simiao

Energy science is becoming an increasingly important multi-disciplinary area, for not only addressing the worldwide energy crisis, but also realizing desired power sources with advanced features for portable electronic devices and sensor networks. Very recently, based on triboelectric effect and electrostatic induction, a fundamentally new technology, triboelectric nanogenerator, has been demonstrated which shows unique merits. But so far, the main limitation for continuing optimizing their output performance is a lack of fundamental understanding of their core working mechanism. In this thesis research, we first unveil the fundamental theory and output characteristics of triboelectric nanogenerators. Then, we apply the developed theory to the TENG-based self-powered system design. We have developed the first genuine self-powered system to meet mW requirement of personal electronics. The system includes a multilayered TENG, a power management circuit with 60% total efficiency, and a low leakage energy storage device. Our power management circuit provides the total efficiency that is about two magnitudes higher than the traditional direct charging. And the total system performance is 330 times higher than the state-of-art designs. Driven by palm tapping, this power unit can provide a continuous DC electricity of 1.044 mW on average power in a regulated and managed manner that can be universally applied as a standard power source for continuously driving numerous conventional electronics, such as a thermometer, a heart rate monitor (electrocardiograph/ECG system), a pedometer, a wearable electronic watch, a scientific calculator, and a wireless radio-frequency communication system. Our study demonstrates the first power unit that utilizes widely accessible biomechanical energy source to sustainably drive a broad range of commercial mobile and wearable electronic devices. This self-charging unit is a paradigm shift towards infinite-lifetime energy sources that can never be achieved solely by batteries.
Antimony doped p-type zinc oxide for piezotronics and optoelectronics

(Georgia Institute of Technology, 2015-11-04) Pradel, Ken Charles

Zinc oxide is a semiconducting material that has received lot of attention due to its numerous proeprties such as wide direct band gap, piezoelectricity, and numerous low cost and robust methods of synthesizing nanomaterials. Its piezoelectric properties have been harnessed for use in energy production through nanogenerators, and to tune carrier transport, birthing a field known as piezotronics. However, one weakness of ZnO is that it is notoriously difficult to dope p-type. Antimony was investigated as a p-type dopant for ZnO, and found to have a stability of up to 3 years, which is completely unprecedented in the literature. Furthermore, a variety of zinc oxide structures ranging from ultra-long nanowires to thin films were produced and their piezotronic properties were demonstrated. By making p-n homojunctions using doped and undoped ZnO, enhanced nanogenerators were produced which could see application in gesture recognition. As a proof of concept, a simple photodetector was also derived from a core-shell nanowire structure. Finally, the ability to integrate this material with other semiconductors was demonstrated by growing a heterojunction with silicon nanowires, and investigating its electrical properties. All this work together lays the foundation for a fundamentally new material that could see application in future electronics, optoelectronics, and human-machine interfacing.
Patterned thin-film triboelectric generator for harvesting micro-meso scale ambient energy for kinematic sensing

(Georgia Institute of Technology, 2015-07-20) Jing, Qingshen

Harnessing random micro-meso scale ambient energy (M2SAE), which is widely available in human motions, wind driven vibrations, water surface fluctuations, etc., is not only clean and sustainable, but it also enables self-powered sensors and devices to be realized. In my research, I have fabricated a case-encapsulated triboelectric generator (cTENG) based on the principles of sliding electrification for harvesting M2SAE from reciprocating motions. Patterned with multiple sets of grating electrodes and lubricated with polytetrafluoroethylene (PTFE) nanoparticles, cTENG generated an average effective output power of 12.2 mW over a 140 kΩ external load and a power density of 1.36 W/m2 at a sliding speed of 1 m/s. The cTENG can also be triggered by direct-applied forces, as well as, inertia forces to effectively capture ambient energy from vibrations of large amplitudes and low frequencies such as those arising in human motions and water surface fluctuations. Based on the success of the patterned cTENG, I have built a self-powered velocity sensor for either rectified linear or rotary motion by sourcing the energy from the triboelectric generator. Employing alternating Kapton-copper strips arranged in a spiral configuration wrapped on the inner and outer surfaces of two concentric cylinders, voltage assays for linear and rotary motions can be measured without the need for an external power source. The triboelectric generated output signals when integrated with a digital circuit and a microcontroller unit can be directly processed into remarkably stable, macro-scale output signals for measurements of (0.1-0.6) ms-1 +_ 0.5% for linear velocities and (300-700) rpm +_ 0.9% for rotary velocities. I have also fabricated a self-powered, thin-film motion direction sensor by harvesting the operational energy from a close-proximity triboelectrification of two surfaces in relative reciprocation. The mover made by coating a thin polytetrafluoroethylene film with a 2-column, specially arranged array of copper electrodes and the stator is made by coating the top and bottom surfaces of a thin polyimide film with a 2-column aligned array of copper electrodes placed in an alternating pattern. As the mover traverses over the stator, the electrodes in the mover actively generate electric signals of ±5 V to attain a peak power density of ≥ 65 mW/m2 at speeds of 0.3 ms-1. The prototype can be extend for 2-D motion direction sensing. The highly pliable sensor can be easily bent to spread over curved and uneven surfaces. Finally, I have demonstrated a quasi-static angular positioning sensor based on 4-channel encoded pattern on the electrification surface. The sensor consists of a rotator designed with 4-channel coding Cu foil material and a stator including electrodes covered with FEP (fluorinated ethylene propylene) ﬁlm. Due to coupling effect of triboelectriﬁcation and electrostatic induction, the sensor generates electric output signals in response to mechanical rotating motion of an object mounted with the sensor. The sensor can read and remember the absolute angular position regardless being continuously monitored or segmented monitored. Velocity and acceleration can be calculated as well. Under a rotation speed of 100 rad min-1, the output voltage of the sensor reaches as high as 60 V. Angular resolution of 22.5° is achieved and can be further improved by increasing the number of channels. Triggered by the output voltage signal, the rotating characteristics of the steering wheel can be real-time monitored and mapped by being mounted to the sensor. My work represents the first successful attempt in harvesting M2SAE using a patterned triboelectric generator and then, using the harvested ambient energy to drive a kinematic sensor that is integrated with a commercial digital circuit for a dual-mode speed and direction sensing. I believe my pioneering demonstration of the applied triboelectric technology will have a huge impact in the industrial commercialization of self-powered devices and sensors.
Nanogenerators for mechanical energy harvesting and self-powered sensor networks

(Georgia Institute of Technology, 2015-06-26) Lin, Long

Energy crisis and internet of things have attracted long term interests due to rapid development of modern society. In this regard, the invention of nanogenerators provide us new insights on scavenging wasted mechanical energy from the ambient environment, and convert kinetic agitations into electricity for powering electronic devices. Piezoelectric nanogenerators are based on the piezoelectric property of semiconductor nanowires. Vertical integration, proper selection of materials, and surface modifications have been applied to enhance its output performance. Triboelectric nanogenerators (TENG), on the other hand, work on the basis of contact electrification and electrostatic induction. Four fundamental working modes have been developed to accommodate different types of mechanical motions. A high output power density of 35.6 W/m2, an excellent energy conversion efficiency of up to 55%, and terrific output stability of over 300,000 cycles have been accomplished with the state-of-art TENG devices. The niche applications of the TENGs have been demonstrated for powering portable electronics toward the goal of fully-integrated self-powered system. Since the nanogenerators are enabled to convert mechanical input into electrical output signals, the information of mechanical stimuli (amplitude and frequency) can be retrieved through analyzing the output performance of the nanogenerators. In this way, nanogenerators were employed as self-powered/active sensors without an external power supply, and multiple functions could be achieved including pressure detection and motion sensing. Both static and dynamic pressure sensing was realized using the open-circuit voltage and short-circuit current from the TENG, respectively. A high sensitivity of 0.31 kPa-1 and a low detection limit of 2.1 Pa were also fulfilled. The integration of pressure sensor array for tactile imaging was further demonstrated for its potential application in electronic skin, human-machine interface, and security monitoring.